Displays may be broadly classified into one of two types: emissive (e.g., CRTs and plasma display panels (PDPs)) or non-emissive. This latter family, to which liquid crystal displays (LCDs) belong, relies upon an external light source, with the display only serving as a light modulator. In the case of liquid crystal displays, this external light source may be either ambient light (used in reflective displays) or a dedicated light source (such as found in direct view displays).
Liquid crystal displays rely upon three inherent features of the liquid crystal (LC) material to modulate light. The first is the ability of the LC to cause the optical rotation of polarized light. Second, is the ability of the LC to establish this rotation by mechanical orientation of the liquid crystal. The third feature is the ability of the liquid crystal to undergo this mechanical orientation by the application of an external electric field.
In the construction of a simple, twisted nematic (TN) liquid crystal display, two substrates surround a layer of liquid crystal material. In a display type known as Normally White, the application of alignment layers on the inner surface of the substrates creates a 90.degree. spiral of the liquid crystal director. This means that the polarization of linearly polarized light entering one face of the liquid crystal cell will be rotated 90.degree. by the liquid crystal material. Polarization films, oriented 90.degree. to each other, are placed on the outer surfaces of the substrates.
Light, upon entering the first polarization film, becomes linearly polarized. Traversing the liquid crystal cell, the polarization of this light is rotated 90.degree. and is allowed to exit through the second polarization film. Application of an electric field across the liquid crystal layer aligns the liquid crystal directors with the field, interrupting its ability to rotate light. Linearly polarized light passing through this cell does not have its polarization rotated and hence is blocked by the second polarization film. Thus, in the simplest sense, the liquid crystal material becomes a light valve, whose ability to allow or block light transmission is controlled by the application of an electric field.
The above description pertains to the operation of a single pixel in a liquid crystal display. High information type displays require the assembly of several million of these pixels, which are referred to as sub pixels, into a matrix format. Addressing, or applying an electric field to, all of these sub pixels while maximizing addressing speed and minimizing cross-talk presents several challenges. One of the preferred ways to address sub pixels is by controlling the electric field with a thin film transistor located at each sub pixel, which forms the basis of active matrix liquid crystal display devices (AMLCDs).
The manufacturing of these displays is extremely complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need have their physical dimensions tightly controlled. The downdraw sheet or fusion process, described in U.S. Pat. Nos. 3,338,696 (Dockerty) and 3,682,609 (Dockerty), is one of the few capable of delivering such product without requiring costly post forming finishing operations, such as lapping and polishing. Unfortunately, the fusion process places rather severe restrictions on the glass properties, requiring relatively high liquidus viscosities, preferably greater than 200,000 poises.
Typically, the two substrates that comprise the display are manufactured separately. One, the color filter plate, has a series of red, blue, green, and black organic dyes deposited on it. Each of these primary colors must correspond precisely with the pixel electrode area of the companion, active, plate. To remove the influence of differences between the ambient thermal conditions encountered during the manufacture of the two plates, it is desirable to use glass substrates whose dimensions are independent of thermal condition (i.e., glasses with lower coefficients of thermal expansion). However, this property needs to be balanced by the generation of stresses between deposited films and the substrates that arise due to expansion mismatch. It is estimated that an optimal coefficient of thermal expansion is in the range of 28-33.times.10.sup.-7 /.degree. C.
The active plate, so called because it contains the active, thin film transistors, is manufactured using typical semiconductor type processes. These include sputtering, CVD, photolithography, and etching. It is highly desirable that the glass be unchanged during these processes. Thus, the glass needs to demonstrate both thermal and chemical stability.
Thermal stability (also known as thermal compaction or shrinkage) is dependent upon both the inherent viscous nature of a particular glass composition (as indicated by its strain point) and the thermal history of the glass sheet as determined by the manufacturing process. U.S. Pat. No. 5,374,595, disclosed that glass with a strain point in excess of 650.degree. C. and with the thermal history of the fusion process will have acceptable thermal stability for active plates based both on a-Si thin film transistors (TFTs) and super low temperature p-Si TFTs. Higher temperature processing (such as required by low temperature p-Si TFTs) may require the addition of an annealing step to the glass substrate to ensure thermal stability.
Chemical stability implies a resistance to attack of the various etchant solutions used in the manufacture processes. Of particular interest is a resistance to attack from the dry etching conditions used to etch the silicon layer. To benchmark the dry etch conditions, a substrate sample is exposed to an etchant solution known as 110BHF. This test consists of immersing a sample of glass in a solution of 1 volume of 50 wt. % HF and 10 volumes 40 wt. % NH.sub.4 F at 30.degree. C. for 5 minutes. The sample is graded on weight loss and appearance.
In addition to these requirements, AMLCD manufacturers are finding that both demand for larger display sizes and the economics of scale are driving them to process larger sized pieces of glass. Current industry standards are Gen III (550 mm.times.650 mm) and Gen III.5 (600 mm.times.720 mm), but future efforts are geared toward Gen IV (1 m.times.1 m) sizes, and potentially larger sizes. This raises several concerns. First and foremost is simply the weight of the glass. The 50+% increase in glass weight in going from Gen III..5 to Gen IV has significant implications for the robotic handlers used to ferry the glass into and through process stations. In addition, elastic sag, which is dependent upon glass density and Young's Modulus, becomes more of an issue with larger sheet sizes impacting the ability to load, retrieve, and space the glass in the cassettes used to transport the glass between process stations.
Accordingly, it would be desirable to provide a glass composition for display devices having a low density to alleviate difficulties associated with larger sheet size, preferably less than 2.45 g/cm.sup.3 and a liquidus viscosity greater than about 200,000 poises. In addition, it would be desirable for the glass to have thermal expansion between about 28-35.times.10.sup.-7 /.degree. C., and preferably between about 28-33.times.10.sup.-7 /.degree. C., over the temperature range of 0-300.degree. C. Furthermore, it would be advantageous for the glass to have a strain point greater than 650.degree. C., and for the glass to be resistant to attack from etchant solutions.